The Influence of Different Wake Profiles on Losses in a Low Pressure Turbine Cascade

Florian Hammer,Neil Sandham,Richard Sandberg

doi:10.3390/ijtpp3020010

Abstract

Large eddy simulations were carried out in order to investigate the influence of unsteady incoming wakes with different profiles on the loss mechanisms of the high lift T106Alinear low-pressure turbine (LPT) cascade. Bars placed upstream of the LPT blade were set into rotation around their axis, thus generating circulation, as well as asymmetrical wake profiles. Three different rotation rates were simulated, yielding different wake parameters that were then compared to an actual turbine blade wake profile. Whereas the commonly-used non-rotating bars generated wakes with turbulent kinetic energy levels several times higher than that of an actual blade wake, the case with counter-clockwise rotation led to more rapid wake mixing. All three wakes were able to trigger boundary layer transition and thus intermittently prevent separation on the suction surface. However, the weaker the wakes, the larger and longer lasting the separation bubbles became, and an increase in profile losses could be observed. Interestingly, the configuration with the weakest wake and the largest separation bubble resulted in a reduction of the overall LPT loss.

Highlights

The low-pressure turbine (LPT) in a jet engine makes up 20–30% of its total weight, and its dimension is restricted by the diameter of the jet engine casing
Skewed and much weaker wake profiles are achieved by the counter-clockwise rotating bars, which are more similar in shape to an actual blade wake
By setting the wake-generating bars into rotation, it was possible to achieve more similar wakes compared to an actual blade wake in terms of wake width

Summary

Introduction

The low-pressure turbine (LPT) in a jet engine makes up 20–30% of its total weight, and its dimension is restricted by the diameter of the jet engine casing. The rotational speed of the LPT and the prevalent flow velocities are determined by the operational range of the fan, which is driven by the turbine. In the Reynolds number range in which an LPT operates, boundary layer transition and separation play an important role and have to be taken into account in the design process. Owing to the higher loadings, the boundary layer on the suction side of the blade is exposed to large adverse pressure gradients leading to unsteady transitional boundary layers [2]. Hodson and Howell [1] state that as the flow on the pressure side still accelerates in the direction of the trailing edge, the boundary layer remains laminar in most cases. A laminar separation bubble develops on the suction surface on the rear part of the blade due to the adverse pressure gradient. The separation bubble is highly sensitive to unsteady incoming wakes and disturbed flow in the LP turbine [1]

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